A calorie is defined as a “unit of energy”. This unit of energy must create enough heat to raise the temperature of 1 liter of water by 1 degree Celsius.
When you eat your body either burns it for immediate use, or stores it for use later on.
Calories can not disappear, they can only transform. Upon digestion of a calorie, it is dispersed as fuel for heat, energy, sound, and movement.
Any unused calorie is transformed into chemical energy and gets stored as fat.
An individual with high muscle mass will continue to burn calories even at rest because the more muscle a person has the more energy is required to maintain it.
An individual with high-fat percentage has no need to burn incoming calories because the excess calories stored as fat serve as a preordained energy facility for the body.